The present invention relates to long persistent phosphors and methods for their fabrication and use, as well as relating to phosphorescent articles incorporating the phosphors of the invention. The phosphors of the invention include alkaline earth aluminates, alkaline earth silicates, and alkaline earth aluminosilicates.
A wide variety of phosphors are known to the art. Persistent phosphorescing materials, such as ZnS:Cu, Co, ZnCdS:Cu and CaSrS:Bi, have been used for many years.
However, far fewer types of long persistence phosphors have been developed. These phosphors show a persistent afterglow emission or phosphorescence which can last for up to ten or more hours following a relatively brief period of ultraviolet excitation. Long persistence phosphors potentially have many technical applications.
Long persistent alkaline earth metal aluminate phosphors have been reported. Recent interest has centered on various aluminates doped with two rare earth ions, such as SrAl2O4:Eu2+, Dy3+, a green phosphor. These compounds are attractive from the point of view of chemical stability and low toxicity. A number of these materials have been commercialized for signing and novelty applications.
Strong green luminescence from Eu2+-doped SrAl2O4 was reported by H. Lange in Belgian patent 1,347,45 and U.S. Pat. No. 3,294,696. Efficient luminescence in the spectral range 450-520 nm was also reported from Eu2+-doped CaAl2O4, MgAl2O4, BaAl2O4, and SrAl2O4 and their counterparts using alkaline earth cationic combinations. (F. C. Palilla, A. K. Levine and M. R. Tomkus (1968) J. Electrochem. Soc. 115:642).
Long lasting and more efficient phosphorescence was reported in Eu2+ doped SrAl2O4 synthesized with excess alumina which results in formation of trapping centers associated with the Sr2+ vacancy (Abbruscato et al. (1971) J. Electrochem. Soc. 118:930).
Improved long persistence phosphors of certain alkaline earth aluminates were reported by T. Matsuzawa, Y. Aoki, N. Takeuchi and Y. Murayama (1996) J. Electrochem. Soc. 143(8):2670, and in U.S. Pat. No. 5,424,006. The brightness and persistence time of SrAl2O4:Eu2+ was improved by co-doping various trivalent rare earth ions to produce appropriate trapping centers. The best result was reported for co-doping Dy+3 with Eu2+ into SrAl2O4 and Nd+3 with Eu2+ into CaAl2O4 to get long persistent green and purple emission, respectively. U.S. Pat. No. 5,424,006 also reports phosphors in which Mg2+ is substituted for Sr2+ in SrAlO4:Eu2+, Dy3+.
U.S. Pat. No. 6,267,911 (Yen et al.) discloses long persistent alkaline earth aluminate green phosphors of the general form MkAl2O4 activated by Eu2+ in combination with at least one trivalent metal ion such as Dy3+. M is an alkaline earth element such as Ca, Mg, Ba, or Sr. In these phosphors, k=1−2x−2y where x ranges from about 0.0001 to about 0.05 and y ranges from about x to about 3x.
U.S. Pat. No. 6,117,362 (Yen et al.) discloses long persistent alkaline earth aluminate blue phosphors of the general form MO.mAl2O3 activated by Eu2+ in combination with at least one trivalent metal ion such as Dy3+. M is an alkaline earth element such as Ca, Mg, Ba, or Sr. In this formula, m is a number ranging from about 1.6 to about 2.2.
EP published application 765,925 (Moriyama et al.) reports Eu2+-activated strontium aluminate phosphors in which part of the Sr2+ of the host is replaced with Pb, Dy or Zn ions. The zinc-doped materials are reported to display enhanced brightness and persistence compared to SrAl2O4:Eu2+, Dy3+.
EP published application 710,709 (Murayama et al.) reports phosphors of matrix M1-xAl2O4-x where M is at least one metal selected from calcium, strontium, barium and, optionally, magnesium, and x is a number not equal to 0. The matrix comprises europium as an activator and a co-activator elected from a rare earth metal, manganese, tin or bismuth.
JP Patent 76031037 (1976, Tokyo Shibaura Electric Co.) reports blue-emitting phosphors containing barium (or calcium or strontium)-potassium (or sodium) aluminates activated with europium and manganese.
JP Patent 94029417 (1994, Matsushita Electronics) reports a strontium aluminate phosphor activated with europium modified by incorporation of yttrium oxide.
JP Patent 94029416 (1994, Matsushita Electronics) reports a europium activated barium aluminate phosphor containing yttrium oxide to enhance phosphorescence.
Zlotnikova et at. (1990) Ukr. Khim Zh. (Russ. Ed.) 56(11):1148-1151 (Chem. Abst. (1991) 115:37798k) reports composition dependence of catho-luminescent properties of a Dy-doped SrAl2O4—Sr Al4O7 system.
T. R. Kutty et al. (1990) Mater. Res. Bull. 25:1355-1362 reports luminescence of Eu2+ in strontium aluminates prepared by the hydrothermal method. Blue to green luminescent phosphors of general formula SrnAl2O3+n where n≦1 or less are reported. The reference also reports the preparation of certain aluminoborates.
B. Smets et al. (1989) J. Electrochem. Soc. 136(7):2119-2123 reports blue-emitting phosphors: 2SrO.3Al2O3:Eu2+ and 1.29 (Ba, Ca)O, 6Al2O3:Eu2+. In the background section of the reference the authors refer to an earlier report of blue-green emitting phosphors 4SrO.7Al2O3:Eu2+ and BaO.4Al2O3:Eu2+ which could be synthesized only in the presence of small amounts of B2O3.
Chemekova et al. (1977) Terzisy Dokl. Uses. Soveshch. Rostu. Krist. 5th 2:184-185 (Chem. Abst. (1980) 93:85423h) reports synthesis of single crystals in the calcium oxide-alumina system. Addition of europium is said to produce phosphors.
Japanese published application number 08-151573 (1996, Nichia Chem. Ind. Ltd.), reports a calcium aluminate fluorescent substance additionally containing boron and optionally containing phosphorus. The aluminates also contain Eu, Nd and Mn, although Nd and Mn may be present in quantities as small as 0.00005. Drawing 1 of the published application shows an emission spectrum of a calcium aluminate containing Eu, Nd, Mn and boron. Immediately after excitation of the material is stopped, the material has emission peaks at 440 nm and 550 nm (curve a). The 440 peak is stated to be due to Eu2+ and the 550 nm peak due to Mn2+. Significantly, the amount of 550 nm luminescence decreases (curve b) after 20 minutes.
Persistent and long-persistent alkaline earth metal silicate phosphors formulated with boron or phosphorus have also been reported. U.S. Pat. No. 6,093,346 to Xiao et al. reports europium activated silicates of the formula: aMO.bM′.cSiO2.dR:EuxLny wherein M represents one or more elements selected from the group consisting of Sr, Ca, Ba, and Zn; M′ represents one or more elements selected from the group consisting of Mg, Cd, and Be; R represents one or two components selected from B2O3 and P2O5; Ln represents one or more elements selected from the group consisting of Nd, Dy, Ho, Tm, La, Pr, Tb, Ce, Mn, Bi, Sn, and Sb; and wherein a, b, c, d, x, and y represent mole coefficients wherein 0.6≦a≦6, 0≦b≦5, 1≦c≦9, 0≦d≦0.7, 0.00001≦x≦0.2, and 0≦y≦0.3. The materials are stated to be capable of having an emission peak ranging from 450 nm to 580 nm when excited by short wavelength light, with the afterglow color being blue, bluish green, green, greenish-yellow, or yellow.
Long persistent alkaline earth metal alumino-silicates which are typically formulated with an alkaline halide, ammonium halide salt, and/or ammonium phosphates have also been reported. WO 02/083814 A1 (Su et al.) describes materials of the composite formula MO.aAl2O3.bSiO3.cL:fX, where M is SrO, CaO, BaO, and/or MgO, L is a mineralizer containing an alkaline halide, and/or an ammonium halide salt, and/or ammonium phosphates, and X is a rare-earth element activator.